E.m.g./moment relationships in neck muscles during isometric cervical spine extension

The aim of the study was to describe the e.m.gJmuscular moment (torque) relationships for neck and shoulder muscles during cervical spine extension. Ten healthy women participated. Their average age was 25·6 years. The neck extension muscular moments exerted were measured isometrically in neutral and in flexed lower-cervical spine positions with a strain gauge connected to a sling around the back of the head. The moment about the bilateral axis of the C7 T1 spinal motion segment was calculated as the moment balancing the sum of the moment of the sling resistance force and the moment caused by the gravity forces of the head and neck. Video images were analysed for moment arm length measurements. Simultaneously with the force recording, the e.m.g. activity was recorded through surface electrodes applied unilaterally at six locations in the neck and shoulder region. The rectified low-pass filtered e.m.g. signals were normalized against the highest activity level obtained during a series of standardized isometric test contractions. The results show that a non-linear e.m.g./moment relationship exists in axial musculature of the cervical spine, with a slightly higher increase in e.m.g. levels at high muscular moment values.